1. Field of the Invention
The present invention relates to a driving device of a vehicle that includes a differential device having a differential function and thereby operating as a speed change device, i.e., a transmission, and particularly to the art of assuring that the driving device can employ, e.g., an electric motor having a small capacity.
2. Related Art Statement
There is known a driving device of a vehicle that includes a differential device that distributes an output of an engine to a first electric motor and an output shaft; and a second electric motor that is provided between the output shaft of the differential device, and drive wheels of the vehicle. An example of a driving device of a hybrid vehicle is disclosed by each of Japanese Patent Application Publication No. 2000-2327 and Japanese Patent Application Publication No. 2000-346187. In the hybrid-vehicle driving device, the differential device is constituted by, e.g., a planetary gear set, and it mechanically transmits, owing to a differential function of the planetary gear set, a main portion of driving power of the engine to the drive wheels, and electrically transmits the remaining portion of the power via an electric path from the first electric motor to the second electric motor. Thus, the differential device functions as a transmission whose speed ratio is electrically changeable, for example, an electric CVT (continuously variable transmission). The differential device is controlled by a control device so that the vehicle can run while the engine is kept in an optimum operating state. Thus, a fuel consumption rate of the vehicle is improved.
Meanwhile, usually, a vehicle can stably turn in response to driver's operation of a steering member such as a steering wheel. However, depending upon a running condition such as running on a road surface having a low frictional resistance, running at a high speed, or turning at a high speed, or an external cause, a behavior of the vehicle may become unstable when the vehicle is turning. The unstable behavior of the vehicle is, for example, a state in which rear wheels of the vehicle lose more grip of road surface than front wheels thereof and accordingly the rear wheels tend to slip in a lateral direction thereof, or a state in which the front wheels lose more grip of road surface than the rear wheels and accordingly the front wheels tend to slip in the lateral direction. When the rear wheels tend to slip in the lateral direction, so-called “over-steering” tends to occur, that is, the vehicle tends to turn by an angle excessively greater than an angle corresponding to an amount of driver's operation of the steering member. When the front wheels tend to slip in the lateral direction, so-called “under-steering” tends to occur, that is, the vehicle tends to turn by an angle excessively smaller than the angle corresponding to the amount of operation of the steering member.
There is known a control device of a vehicle that employs a vehicle behavior stabilization control means or device that stabilizes a behavior of the vehicle by controlling, when the behavior of the vehicle becomes unstable, an output torque of a driving-power source (i.e., a driving-power-source torque) or a braking force applied to each drive wheel, and thereby controlling a torque of the each drive wheel. An example of the vehicle control device is disclosed by Japanese Patent Application Publication No. 2003-194209. The disclosed vehicle control device employs, as the vehicle behavior stabilization control means, a device that is called “VSC (vehicle stability control) system” and stabilizes a behavior of the vehicle when the vehicle is turning. If over-steering or under-steering tends to occur when the vehicle is turning, the VSC system stabilizes the vehicle by lowering an output torque of an engine (i.e., an engine torque) and applying a braking force to front and/or rear wheels, thereby producing a rear-wheel-lateral-slip restraining moment or a front-wheel-lateral-slip restraining moment.
As a different sort of vehicle behavior stabilization control means than the VSC system, there is well known a device that is called “ABS (antilock brake system)”. When a vehicle is braked, the ABS appropriately controls a braking force applied to a drive wheel or wheels, so as to prevent locking of the drive wheel or wheels and thereby assure that the vehicle can enjoy an excellent braking performance and thereby maintain high stability and steerability.